This invention relates to a novel extrusion apparatus and particularly to an improved design of the extruder barrel and screw to be used in a single screw extruder fed with solid material rather than melted material.
The efficiency of an extruder screw is determined by the manner in which the material is conveyed by the screw under controlled temperature and pressure conditions with sufficient mixing to produce a homogeneous melt at the end of the screw.
In the single screw extrusion process, the chips or particulate solids are conveyed forward by the screw by way of friction between the chips, the barrel, and the screw. The process is such that it is preferred that the coefficient of friction between the chips and the barrel is greater than that between chips and the screw. Conditions can exist where the chips will slip against the barrel and rotate with the screw and will not advance. The most effective method of overcoming this problem is to design the extruder in such a manner as to ensure sufficient friction within the feed section. One such method involves altering the normally smooth barrel wall within the feed section (usually 2 to 3 barrel diameters). Such designs would include grooves, splines, tapered splines, randomly located indentations or barriers.
In a plasticizing extruder screw, there are at least three major geometrical sections: feed, transition, and metering. The three major geometrical sections correspond more or less to the three functional sections of the screw. These functions are solids conveying, melting and melt conveying. Solids conveying occurs in the feed section, although some melting may begin in this section. The melting occurs mainly in the transition section, although the melting is not necessarily completed at the end of this section. The melt conveying occurs in the metering section, although melting can continue in this section.
While the geometrical sections of the extruder screw are fixed, the functional sections are not fixed. These boundaries will shift, depending on the material extruded, the operating conditions, and other parameters. Sometimes, other geometrical sections are included to perform special functions such as mixing, devolatilizing and the like.
Other variations of extruder screws include those containing internal heat pipes for heat transfer from metering section to feed section, a decompression section to control melt temperature, an internal screw for melt removal, or an internal screw for solids recirculation. The "Barr" screw (U.S. Pat. No. 3,698,541), uses, for example, a double flight to separate melted resin from unmelted granules in an extrusion process and this results in an improved extrusion over conventional square pitch extrusion screws.
Prior art screw designs vary considerably according to the processing application required. The screw usually has one or two metal ridges or "flights" wrapped like a large screw thread around a cylindrical "core". The helix angle of the flight is conventionally constant throughout the three sections with the angle of 17.66.degree. (a "square pitch," where pitch equals diameter) being very common; but it is also known in the art to change the helix angle using one angle in a given section and another angle in another section.
In almost all applications, the efficiency of an extruder can be improved by proper design. A mathematical description of solid conveying in the feed section is discussed by W. H. Darnell and E. A. J. Mol ("Solids Conveying in Extruders" SPE Journal, April 1956) and others. The Darnell/Mol model is based on solid-to-solid friction between the solid plug and the surrounding metal surfaces (barrel, screw core, trailing, and leading screw flight).
Optimization of the geometry of the feed section is difficult because it depends on the values of coefficients of the friction. The coefficients of friction are not constant, but change with temperature, pressure, velocity, and the like. Therefore, an average value of the coefficient is usually used for simplicity. For most plastics, the coefficient of friction is approximately 0.25 to 0.5. Using the values 0.25 to 0.5 for coefficient of friction and the solid conveying theory, curves can be determined on output vs. helix angle. Maximum outputs will be obtained at certain values of the helix angle depending on the value of the coefficient of friction, the diameter of the screw, and the channel flight height in the feed section.
A mathematical model of the melting in the extruder screw is given by Tadmor and Klein (see Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold Company, 1970, Chapter 5). This model can be used only with a sophisticated computer program because the calculation is made by an iterative process. The usefulness of this model lies mostly in the simulation of extruder screw with a constant helix angle and uninterrupted flights. In other screw designs, the usefulness of the model is questionable.
Ernest C. Bernhardt (Processing of Thermoplastic Materials, Reinhold Publishing Corporation, New York, 1959, page 212) discusses optimization of the melt flow through the metering section.
Until now, no one has applied all the optimization theories above discussed to a single extruder screw in the manner described herein, for the resultant screw dimensions fall outside the presently accepted norms. For example, the flight height ratio between the feed and the metering sections is much higher than present conventional screw design theories accept.